In the prior art, it is known to deposit a material onto a substrate via electron beam induced deposition (EBID) and ion beam induced deposition (IBID). According to the known method, a substrate is placed in the evacuable specimen chamber of a charged particle beam apparatus—typically either an electron beam (E-beam) system or a focused ion beam (FIB) system. The charged particle (or other) beam is applied to the substrate surface in the presence of a deposition gas, often referred to as a precursor gas. A layer of the precursor gas adsorbs to the surface of the work piece. The thickness of the layer is governed by the balance of adsorption and desorption of the gas molecules on the substrate surface, which in turn depends on, for example, the partial gas pressure (determining how many molecules are adsorbed per second), and the sticking coefficient (describing how long, on average, a molecule is adsorbed to the surface). The resultant layer is typically formed of less than one to several mono-atomic layers.
When the charged particle beam irradiates the substrate with the adsorbed layer of precursor gas, secondary electrons are emitted from the substrate. These secondary electrons cause a dissociation of the adsorbed precursor gas molecules. Part of the dissociated precursor material forms a deposit on the substrate surface, while the rest of the precursor gas particle forms a volatile by-product and is pumped away by the vacuum system of the apparatus.
Beam induced deposition is used in a wide variety of applications for depositing a material onto a target surface of a substrate such as a semiconductor wafer or magnetic storage media. The materials are deposited for a variety of reasons such as to form patterned thin-film surfaces, protective coatings for semiconductor feature characterization and analysis, or to “weld” small samples, such as TEM samples, to a manipulator or sample holder (as described in more detail below). Many combinations of gasses, substrates, and beam types can be used to achieve a variety of deposition schemes. The particular material to be deposited will usually depend on the application, underlying target surface, and how the material reacts with the target surface. Similarly, a variety of beam types can be used to generate secondary electrons. These include ion, electron, and laser beams.
A disadvantage of known beam induced deposition methods is that they do not work well at cryogenic temperatures (below −50° C., more specifically below −130° C.). A number of applications, including the preparation of TEM samples of biological materials, require samples to be cooled to cryogenic temperatures. For example, to preserve the structural integrity of biological samples in the low-pressure environment of a TEM sample chamber, samples are often vitrified to avoid dehydration. Vitrification involves the process of cooling the sample so rapidly that the water molecules within the sample do not crystallize, but stay in an amorphous or vitrified state that does little or no damage to the sample structure. Vitreous ice is also featureless and does not form destructive ice crystals. The low temperature of the vitrified sample also may reduce the damage caused by beam electrons during observations, permitting more or longer exposures at higher beam currents for better quality images. To maintain the biological sample in its vitrified state, the temperature must be maintained below −130° C. (the so-named vitrification temperature), if the temperature rises above this level, the crystals change from amorphous to crystalline ice, which destroys the biological information in the cell or tissue.
At cryogenic temperatures, normal EBID/IBID processes do not work. Instead of reaching equilibrium where, for example, at most two monolayers of molecules are adsorbed on the substrate surface, the precursor gas molecules freeze onto the substrate and stay there (—that is: either the sticking coefficient goes to 1 or the lingering period goes to ∞—). This results in a thickening frozen layer of gas molecules covering the surface, making it difficult for the charged particle beam to cause dissociation of the precursor molecules and preventing any deposited material from bonding to the substrate surface resulting in an unreliable bond when used to attach a sample to a manipulator or holder. Further, the typical “volatile by-products” also freeze onto the surface at these low temperatures and may also disturb the formation of the bond. Also, when the sample is at a later stage brought up to a temperature above the melting, sublimation or boiling point of the, then suddenly volatile, by-products, can lead to damage of the intended structure. As a result, it will be hard or impossible to write patterned thin film surfaces on the sample.
Bozso et al., “Electronic Excitation-Induced Surface Chemistry and Electron-Beam-Assisted Chemical Vapor Deposition, Mat. Res. Soc. Symp. Proc., Vol. 158, pp. 201-209 (1990) describes a method of depositing silicon, silicon nitride, silicon oxide, and silicon oxinitride films onto a silicon substrate using low energy EBID at a temperature of approximately 100 K (−173° C.). The deposition method of Bozso, however, is used to separate dissociation reactions caused by electrons from those caused by heat for more precise control over spatial growth and material composition. As a result, Bozso describes adjusting the sample temperatures to achieve a desired deposition. This is not practical for preparation of TEM samples of biological materials since the samples must be maintained below the vitrification temperature to avoid damaging the biological sample. Further, the method described by Bozso is limited to depositing silicon, silicon nitride, silicon oxide, and silicon oxinitride films onto silicon. The use of the silicon substrate plays a dominant role in the absorption and surface chemistry of the reacting molecules. For preparation of TEM samples of biological materials it is desirable that the deposition method work on biological substrates and ice and be able to deposit non-silicon based materials such as carbon. Finally, the method described by Bozso teaches the use of low-energy electron beam excitation (˜200 eV) to induce deposition in order to maintain both spatial and kinetic control. The material deposition rates would thus be impractical for sample welding or even for forming thicker/larger protective layers.
Therefore, there is a need for an improved method of beam-induced deposition that can be used at cryogenic temperatures in much the same way that typical EBID/IBID processes are used at temperatures above −50° C., more specifically above −130° C. There is a further need for an improved method of selecting a suitable precursor gas for beam-induced deposition at cryogenic temperatures.